Metal Oxide-Semiconductor (MOS) devices are key components of integrated circuits. A MOS device includes a gate stack, which further includes a gate dielectric over a semiconductor substrate, and a gate electrode over the gate dielectric. A source region and a drain region are disposed on the opposite sides of the gate stack. The MOS device may be turned on and off by controlling the voltage applied on the gate electrode, so that the source region and the drain region are either electrically disconnected or electrically interconnected.
Conventionally, silicon dioxide was used to form the gate dielectrics. With the evolving of integrated circuits, dielectric materials with high dielectric constants (high-k values) are increasingly used to form the gate dielectrics. The high-k dielectrics may improve the short channel control of the MOS devices. The high-k dielectrics may also reduce gate leakage currents. Hafnium oxide and aluminum oxide are among known high-k dielectric materials.
It is desirable to have high-k materials with the k values higher than the currently used high-k dielectric materials in order to further improve the performance of the MOS devices. However, higher k values also result in problems. For example, with the further increase in the k values, the band-gaps of the high-k dielectric materials reduce, which results in the gate leakage currents of the respective MOS devices to be increased. Therefore, tradeoff has to be made to choose between higher k values accompanied by higher leakage currents and lower k-values accompanied by lower leakage currents.